1. Field of the Invention
The present invention relates generally to nuclear reactors and, more particularly, to a pressurized water nuclear reactor having an improved grid.
2. Description of the Related Art
In most pressurized water nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. These fuel assemblies typically include a plurality of fuel rods held in an organized array by a plurality of grids that are spaced axially along the fuel assembly length and are attached to a plurality of elongated thimble tubes of the fuel assembly. The thimble tubes typically receive control rods, plugging devices, or instrumentation therein. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the ends of the thimble tubes that extend slightly above and below the ends of the fuel rods.
The grids, as is known in the relevant art, are used to precisely maintain the spacing between the fuel rods in the reactor core, resist rod vibration, provide lateral support for the fuel rods and, to some extent, vertically restrain the rods against longitudinal movement. One type of conventional grid design includes a plurality of interleaved straps that together form an egg-crate configuration having a plurality of roughly square cells which individually accept the fuel rods therein. Depending upon the configuration of the thimble tubes, the thimble tubes can either be received in cells that are sized the same as those that receive fuel rods therein, or can be received in relatively larger thimble cells defined in the interleaved straps.
The straps are configured such that the cells each include a pair of relatively compliant springs and plurality of relatively rigid dimples, with the springs and dimples being formed into the metal of the interleaved straps and protruding outwardly therefrom. The springs and dimples of each cell engage the respective fuel rod extending through the cell. Outer straps of the grid are attached together and peripherally enclose the inner straps of the grid to impart strength and rigidity to the grid.
Depending upon the specific configuration of the grid and the straps, the straps may each include one or more mixing vanes formed thereon that facilitate mixing of the water within the reactor and thus promote convective heat exchange between the fuel rods and the water. Since the reactor is designed such that the water moves generally vertically upward through each fuel assembly, the mixing vanes, while promoting mixing of the water, nevertheless impart forces and torques to the grid as a result of the water impinging on the vanes. Such forces and torques on the grid can result in an overall or resultant force, bending moment, or torque applied by the grid to the fuel rods and thimble tubes with resultant stresses and vibrations therebetween. It is thus desired to provide an improved grid design having mixing vanes that are advantageously arranged in such a fashion to result in at most only negligible unbalanced forces, moments, or torques being applied from the grid to the fuel rods and thimble tubes other than forces applied longitudinally to the thimble tubes and compressive and frictional forces applied to retain the fuel rods within the cells.
It is also known in the relevant art that during operation of the reactor the elevated temperatures, pressures, and fluid velocities within the reactor tend to cause vibrations between the grids and the fuel rods. Since the grids support the fuel rods within the fuel cell, such vibrations therebetween can result in fretting of the fuel rods. Such fretting, if sufficiently severe, can result in breach of the fuel rod cladding with resultant nuclear contamination of the water within the reactor. It is thus desired to provide an improved grid having springs and dimples that are designed to minimize fretting wear between the grids and the fuel rods.
In accordance with the foregoing, an improved grid for use in a fuel assembly of a nuclear reactor having a plurality of thimble tubes includes a plurality of first and second straps that are connected with one another in a lattice to define a plurality of cells, with a plurality of mixing vanes being disposed on the first and second straps, and with the mixing vanes being arranged such that the hydraulic forces on the mixing vanes of the grid generally cancel out one another to result in at most only nominal unbalanced forces, moments, and torques being applied to the thimble tubes. The mixing vanes of diagonal quadrants of the grid are generally aligned with imaginary alignment planes that are diagonally disposed on the grid. The grid also includes a plurality of strap members, with each strap member including a spring and a pair of dimples. The spring includes a contoured surface shape having a greater radius of curvature in a relaxed condition than the radius of a fuel rod, and may additionally include a spring embossment. Each dimple includes a contoured surface shape that is similarly configured and that may similarly include a dimple embossment. The spring embossment is mounted on a pair of legs that extend nonlinearly between a first plate and a second plate of each strap member to increase the compliance of the legs.
An aspect of the present invention is to provide a grid for a fuel assembly of a nuclear reactor in which the mixing vanes of the grid are advantageously configured to minimize unbalanced forces, moments, and torques applied between the grids and the fuel rods and thimble tubes of the fuel cell in order to minimize the potential for self-induced vibration.
Another aspect of the present invention is to provide a grid having a plurality of strap members that each include a spring and at least a first dimple, with the springs and dimples each including a contoured surface shape that may include an embossment to reduce fretting wear between the grid and the fuel rods.
Another aspect of the present invention is to provide a strap member of a grid of a fuel assembly of a nuclear reactor having a spring apparatus that includes a spring member and a pair of legs, in which the legs extend nonlinearly between a pair of plates in order to increase the compliance of the spring apparatus.
In accordance with the foregoing, an aspect of the present invention is to provide a grid for use in a fuel assembly of a nuclear reactor, the general nature of which can be stated as including a plurality of first straps aligned with one another, a plurality of second straps aligned with one another, the first and second straps being connected with one another in a lattice to define a plurality of cells. A plurality of mixing vanes are disposed on the first and second straps, with each mixing vane including a free end and a connection end, and with the connection end forming a connection with one of the first and second straps. Each mixing vane includes a longitudinal axis defined thereon that extends between the connection end and the free end. An imaginary first grid axis and an imaginary second grid axis extend across the grid, with the first and second grid axes intersecting one another at a point of intersection to define a first grid quadrant, a second grid quadrant, a third grid quadrant, and a fourth grid quadrant on the grid that are separated from one another by the first and second axes. The cells are each generally of a cell width measured in a direction generally parallel with the second straps and of a cell height measured in a direction generally parallel with the first straps. The longitudinal axes of substantially all of the mixing vanes within a diagonally disposed pair of the first, second, third, and fourth grid quadrants that are spaced at least one-half the cell width from the first grid axis and at least one-half the cell height from the second grid axis extend in a direction generally parallel with one of an imaginary first alignment plane and an imaginary second alignment plane, with the first and second alignment planes each extending substantially perpendicular to a plane including the first and second grid axes. The longitudinal axes of substantially all of the mixing vanes within the other pair of the first, second, third, and fourth grid quadrants that are spaced at least one-half the cell width from the first grid axis and at least one-half the cell height from the second grid axis extend in a direction generally parallel with the other of the first and second alignment planes.
Another aspect of the present invention is to provide a fuel assembly for a nuclear reactor, the general nature of which can be stated as including at least a first grid and at least a first fuel rod engaged with the grid, and in which the general nature of the at least first grid can be stated as including a plurality of first straps aligned with one another, a plurality of second straps aligned with one another, the first and second straps being connected with one another in a lattice to define a plurality of cells. A plurality of mixing vanes are disposed on the first and second straps, with each mixing vane including a free end and a connection end, and with the connection end forming a connection with one of the first and second straps. Each mixing vane includes a longitudinal axis defined thereon that extends between the connection end and the free end. An imaginary first grid axis and an imaginary second grid axis extend across the grid, with the first and second grid axes intersecting one another at a point of intersection to define a first grid quadrant, a second grid quadrant, a third grid quadrant, and a fourth grid quadrant on the grid that are separated from one another by the first and second axes. The cells are each generally of a cell width measured in a direction generally parallel with the second straps and of a cell height measured in a direction generally parallel with the first straps. The longitudinal axes of substantially all of the mixing vanes within a diagonally disposed pair of the first, second, third, and fourth grid quadrants that are spaced at least one-half the cell width from the first grid axis and at least one-half the cell height from the second grid axis extend in a direction generally parallel with one of an imaginary first alignment plane and an imaginary second alignment plane, with the first and second alignment planes each extending substantially perpendicular to a plane including the first and second grid axes. The longitudinal axes of substantially all of the mixing vanes within the other pair of the first, second, third, and fourth grid quadrants that are spaced at least one-half the cell width from the first grid axis and at least one-half the cell height from the second grid axis extend in a direction generally parallel with the other of the first and second alignment planes.
Another aspect of the present invention is to provide a strap member of a grid of a fuel assembly of a nuclear reactor, the nuclear reactor including at least a first fuel rod, the general nature of which can be stated as including a first plate, the first plate including a first dimple, a second plate, the second plate including a second dimple, a spring apparatus including a spring member, a first leg, and a second leg, the first and second legs each extending nonlinearly between the first and second plates, the spring member extending between the first and second legs, the spring member including a spring plate and a pair of spring ligaments, the spring plate being interposed between the spring ligaments, and one of the spring ligaments being connected with the first leg, the other of the spring ligaments being connected with the second leg.
Another aspect of the present invention it to provide a fuel assembly for a nuclear reactor, the general nature of which can be stated as including at least a first fuel rod and at least a first grid, the at least first grid including a plurality of first straps aligned with one another and a plurality of second straps aligned with one another, the first and second straps being connected with one another in a lattice to define a plurality of cells, the at least first fuel rod being disposed in one of the cells, at least one of the first and second straps including a strap member including a first plate, a second plate, and a spring apparatus, the first plate including a first dimple, the second plate including a second dimple, the spring apparatus including a spring member, a first leg, and a second leg, the first and second legs each extending nonlinearly between the first and second plates, the spring member extending between the first and second legs, the spring member including a spring plate and a pair of spring ligaments, the spring plate being interposed between the spring ligaments, and one of the spring ligaments being connected with the first leg, the other of the spring ligaments being connected with the second leg.